Today, appliances or machines for washing dishware or clothes in a liquid medium may be equipped with sensors for measuring the turbidity. It is also possible to have a refrigerator with beverage dispensing system equipped with a sensor for measuring turbidity. As used herein the term turbidity generally refers to the concentration of light-scattering or light-absorbing particles suspended in the liquid medium. If turbidity increases in a fluid then, for a given wavelength, the transmittance generally decreases in dependence of, e.g.:                the wavelength,        the diameter distribution of the suspended particles,        the refractive index of the suspended particles, and        the surface properties of the suspended particles.Settling particles and dissolved matter may also attenuate light travelling in the medium, but are generally not considered to contribute to turbidity. Knowledge of turbidity may be used, inter alia, for adapting various parameters of the washing cycle of the appliance to the degree of contamination of the articles to be washed. This may facilitate more economical use of resources, such as energy, water and detergent.        
Turbidity sensors may be optical sensors positioned in a hydraulic path of the appliance and may measure the optical transmittance of the liquid medium at a certain wavelength. While generally a stable relationship exists between transmittance and turbidity for a specific type of particles, the quality of the turbidity measurements may be a limiting factor for the accuracy. A turbidity sensor may comprise a light source, such as a light-emitting diode (LED) or similar solid-state lighting device, and a light-sensitive element, such as a phototransistor. A portion of the light emitted by the light source can then be received by the light-sensitive element after passing through the liquid medium. By comparing the radiant intensity (radiated power per unit solid angle) I0 of the emitted light and the radiant intensity I of the received light, it is possible to deduce the transmittance of the liquid medium,T=I/I0.  (1)The turbidity of the liquid can then be retrieved via an empirical transmittance-turbidity curve.
Generally, the accuracy of available turbidity sensors is satisfactory only in a narrow range. This shortcoming is partly due to the limited useful range of the light-sensitive elements used in the sensors. Indeed, the correlation between the output of the element and the radiant intensity of received light may degenerate when a saturation level is exceeded. Conversely, for intensities below the sensitivity threshold of the light-sensitive element, the signal may be drowned in noise and may thus be impossible or at least difficult to detect. Accordingly, it is understood that available turbidity sensors have some limitations.